Method for detecting the remaining carrier power on a multi-carrier signal that is QAM-modulated in the 8K mode according to the DVB-T standard

ABSTRACT

The aim of the invention is to detect the remaining carrier power on a multi-carrier signal that is QAM-modulated in the 8K mode according to the DVB-T-standard. Said signal is provided with a central carrier which is situated in the center of the individual carriers and, at times, is a continual and, at times, a scattered pilot. The influence of the remaining carrier power on a carrier that is adjacent to the central carrier is detected and then converted to the central carrier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of PCT/EP00/11437 filed Nov. 17, 2000 whichin turn claims priority to German Patent Application Serial No.:DE10005287.8 filed Feb. 7, 2000.

BACKGROUND

1. Field of the Invention

The invention relates to, and is based on, a method according to theprecharacterising clause of the main claim.

2. Description of the Related Art

The so-called DVB-T standard describes a multi-carrier signal whoseindividual carriers which contain the user data are QAM-modulated, forexample QPSK-, 16-QAM- or 64-QAM-modulated. The number of individualcarriers which are used, and which are indexed by k, depends on the modeof the Fourier transform (FFT) which is used. In the 2 K mode, forexample, k=0 to 1704, and in the 8 K mode k=0 to 6816. The totality ofall the individual carriers transmitted within a fixed time range isreferred to as a symbol, as shown by FIG. 1. Within this carrier range,some of carriers are 2PSK-modulated with additional information aboutthe FFT mode which is used, the QAM order, etc., and these are referredto as TPS (Transmission Parameter Signalling) carriers.

Also present are many unmodulated individual carriers (pilots) whichhave a higher amplitude by the factor 4/3 compared with the mean squareof the amplitudes of the aforementioned carrier types, and a rigid phaseof 0° or 180°, as shown by the constellation diagram according to FIG.2. The phase angle is determined by a pseudo-random number from a PRBS(Pseudo Random Binary Sequence). These pilots are referred to ascontinual pilots, they are found at fixed carrier positions and they areused for coarse adjustment of the frequency of the local oscillator inthe receiver.

Likewise transmitted with increased amplitude and a rigid phase of 0° or180° are the so-called scattered pilots. These are distributed uniformlyover the carrier range and alternate their position within the carrierrange from one symbol to the next, as shown by FIG. 1. Channelestimation at the carrier positions k is possible by interpolation ofthese scattered pilots along the frequency and time axes, and thisinformation obtained by interpolation can then be used for channelcorrection of a perturbed DVB-T signal. With the aid of theconstellation of one or more QAM-modulated individual carriers, it ispossible to derive various transmission parameters which permit anassessment of the quality of the signal separately according tochannel-specific parameters (e.g. signal-to-noise ratio) and influencescaused on the transmission side (e.g. residual carriers or quadratureerrors).

FIG. 3 shows the basic structure of a transmitter-receiver path of asystem operating according to the DVB-T standard. In a data processingstage 1, the digital video signals to be transmitted are processed inthe frequency domain and subsequently converted to the time domain(computer 2) by inverse Fourier transformation (IFFT). The I and Qcomponents produced in this way are sent, after digitising in D/Aconverters 3 and amplification in amplifiers 4, to a quadrature mixture6 in which they are mixed up to the desired output frequency with thecomponents, mutually phase shifted by 90°, of a carrier signal 5 and arere-combined in an adder 7 and are broadcast.

In the receiver, on the input side, a back-conversion of the receivedinput signal from the time domain to the frequency domain in turn takesplace via FFT in the input stage 10, then the aforementioned channelcorrection is carried out by interpolation in a correction stage 11 withthe aid of the scattered pilots, i.e. the received scattered pilots arecorrected in such a way that, on the reception side, they respectivelyassume the ideal position according to FIG. 2 in the I/Q plane. In asubsequent signal evaluation device 12, the received video signals areprocessed further.

A DC component U_(I) or U_(Q) on the transmitter side, as schematicallyindicated in FIG. 3 by the adders 8, can lead to a perturbing residualcarrier which shifts the constellation, in particular, of the centralcarrier of a DVB-T signal, as indicated in FIG. 3 b (FIG. 3 a is theideal constellation of the central carrier). In the case of amulti-carrier signal of this type, the central carrier lies in thebaseband at the frequency zero. In the 2 K mode, it lies at k=852, andin the BK mode it lies at k=3408. In the 2 K mode, this central carrieris modulated with user data and only sometimes constitutes a scatteredpilot, so that the residual carrier power can be determined directlywith reference to it, as is the subject of the prior patent application199 48 383.3, our reference P 22751. In an 8 K mode, the central carrieris not modulated and sometimes constitutes a continual pilot or ascattered pilot. The residual carrier cannot therefore be identifieddirectly with reference to it.

SUMMARY

It is therefore an object of the invention to provide a method withwhich the residual carrier power can be determined in a straightforwardway with reference to the central carrier even in the case of a DVB-Tmulti-carrier signal operating in an 8 K mode.

This object is achieved, on the basis of a method according to theprecharacterising clause of the main claim, by the characterisingfeatures thereof. Two particularly advantageous possibilities for apractical embodiment of this are given by the dependent claims.

By evaluating the influence of the residual carrier power on a carrierneighbouring the central carrier, it is hence possible, according to theinvention, to determine and display the residual carrier power of thecentral carrier even in the case of a DVB-T multi-carrier signal whichis QAM-modulated in the BK mode and in which the central carrier itselftherefore remains unmodulated. This merely requires a conventionalmeasuring receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representation of a QAM-modulated multi-carrier signal inthe 8 K mode according to the DVB-T standard.

FIG. 2 shows the rigid phases of pilots of the signal of FIG. 1 in aconstellation diagram.

FIG. 3 shows the basic structure of a transmitter-receiver path of asystem operating according to the DVB-T standard.

FIG. 4 shows a representation of a method to correct a perturbed DVB-Tsignal with the aid of scattered pilots.

FIG. 5 shows a schematic diagram for calculating the residual carrierusing the correction factor of the central carrier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention will be explained in more detail below with the aid ofschematic drawings with reference to two exemplary embodiments.

In the DVB-T standard according to the ETS300744 specification, there isa scattered pilot at every third carrier position in a transmissionframe, as shown by FIG. 1. The carriers lying in-between are pureuser-data carriers. With the aid of these scattered pilots, it ispossible to correct a perturbed DVB-T signal by a method represented inFIG. 4. By means of this, the user-data carriers lying in-between arealso corrected. The correction system represented in FIG. 4 consists ofa correction branch and a user-data branch. In the correction branch,the pilots of the incoming multi-carrier signal are firstly normalised,i.e. each individual received scattered pilot S_(k) is divided by itsideal position I_(k). In an unperturbed signal, all the scattered pilotshence appear with an amplitude of 1 and a phase angle of 0°.

In an 8 K mode multi-carrier signal, the central carrier has theposition k=3408, as shown by FIG. 1 and FIG. 4. Assuming that anincoming signal in FIG. 4 is perturbed only by a residual carrier V, andnot by any other influences, then all the carriers before the channelcorrection are located in an ideal position and only the central carrierwith the position k=3408 is perturbed by the residual carrier. By meansof the subsequent channel correction in the interpolation filter of thecorrection branch according to FIG. 4, in a known manner byinterpolation between the scattered pilots for the carriers lyingin-between/correction factors are identified, with which the carrierstransmitted in the user-data branch are then corrected in an adderstage. By means of the correction, for example, the neighbouringcarriers k=3406 are multiplied by a complex correction factor x₃₄₀₆, thecarrier k=3407 by a complex correction factor x₃₄₀₇, the carrier k=3409by a complex correction factor x₃₄₀₉, etc. This leads to a rotation andcompression of the respective constellation about its centre. If,however, the interpolation algorithm of the correction branch is known,then it is possible to infer the correction factor x₃₄₀₈ of the centralcarrier by identifying the complex correction factor x_(k) of aneighbouring carrier, and the residual carrier power V (amplitude) canbe determined in this way. It is therefore only necessary, by means ofan additional interpolation filter 15 according to FIG. 4, which has thesame properties as the interpolation filter of the correction branch, toidentify from the corrected carriers the correction factors x_(k)(previously identified in the correction branch) of the carriersneighbouring the central carrier and to calculate the correction factorx₃₄₀₈ of the central carrier by interpolation. For example, by means ofthe recovered correction factor x₃₄₀₇ of the immediately neighbouringuser-data carrier 3407 and by using the same known interpolationalgorithm as was used in the correction branch according to FIG. 2, thecomplex correction factor x₃₄₀₈ of the central carrier can be determinedin a computer 16 and the residual carrier V that is shifting the centralcarrier can then be calculated therefrom, as schematically shown by FIG.5.

With the assumption that the multi-carrier signal is perturbedexclusively by a residual carrier, the following applies for thereceived central carrier before the channel correction:S ₃₄₀₈ =I ₃₄₀₈ +Vwith S₃₄₀₈=received signal of the central carrier, I₃₄₀₈=ideal positionof the central carrier as a scattered pilot, andV=shift due to the residual carrier.

The following furthermore applies:I ₃₄₀₈ =S ₃₄₀₈ /X ₃₄₀₈x₃₄₀₈ is the correction factor determined by the channel correction.

The complex residual carrier is therefore given as follows:V=I+Q=I ₃₄₀₈ (x ₃₄₀₈−1)where I₃₄₀₈ is the predetermined known ideal position of the centralcarrier.

Another possibility for determining the residual carrier in the 8 K modeinvolves evaluating the scattered pilots before the channel correctionrepresented in FIG. 2. In this case, it is assumed that a perturbationin the channel has the same effect on both the central carrier andscattered pilots immediately neighbouring it. By determining theamplitude of one of the scattered pilots neighbouring the centralcarrier, it is hence possible to identify the residual carrier bysubtraction, the scattered pilot immediately neighbouring the centralcarrier (k=3408) has the position 3405 or 3411, as indicated in FIG. 1.With the assumption that the signal is again perturbed only by aresidual carrier, this means that it has no perturbing influence on thescattered pilots, for example k=3405, but rather only on the centralcarrier 3408. The residual carrier can therefore be identified bysubtraction.V=S ₃₄₀₈ −S ₃₄₀₅

Expressed in terms of the neighbouring scattered pilot 3405, theresidual carrier V is defined as follows:V/S ₃₄₀₅ =S ₃₄₀₈ /S ₃₄₀₅−1

In this case, attention should be paid to the differing phase angle ofthe individual scattered pilots. Whereas both of the scattered pilots3405 and 3411 immediately neighbouring the central carrier have a phaseangle of 180°, that is to say the same phase angle as the centralcarrier, the next two scattered pilots 3402 and 3414 have a phase angleof 0°. This must be taken into account appropriately with the sign ±1 inthe above formula.

1. A method for determining the power of a residual carrier perturbing aQAM-modulated multi-carrier signal in the 8 K mode according to theDVB-T (Digital Video Broadcasting for Terrestrial) standard, the methodcomprising: providing a QAM-modulated multi-carrier signal in the 8 Kmode according to the DVB-T standard, said signal including a centralcarrier, said central carrier lying in the middle of individual carriersand being sometimes a continual pilot and sometimes a scattered pilot;using an interpolation algorithm and evaluating scattered pilots in saidmulti-carrier signal to identify correction factors with which channelcorrection of all carriers in said multi-carrier signal is carried out;recovering said correction factor of at least one user-data carrierdirectly or indirectly neighboring said central carrier from saidchannel-corrected multi-carrier signal; determining by interpolation acorrection factor of said central carrier from said correction factor ofsaid at least one user-data carrier; and calculating a power of saidresidual carrier perturbing said central carrier from said correctionfactor of said central carrier.
 2. The method of claim 1 wherein in saidstep of calculating, said power of said residual carrier is calculatedfrom a predetermined ideal position of said central carrier and saidcorrection factor of said central carrier.
 3. A method for determiningthe power of a residual carrier perturbing a QAM-modulated multi-carriersignal in the 8 K mode according to the DVB-T (Digital VideoBroadcasting for Terrestrial) standard, the method comprising: providinga QAM-modulated multi-carrier signal in the 8 K mode according to theDVB-T standard, said signal including a central carrier, said centralcarrier lying in the middle of individual carriers and being sometimes acontinual pilot and sometimes a scattered pilot; and using aninterpolation algorithm and evaluating scattered pilots in saidmulti-carrier signal to identify correction factors with which channelcorrection of all carriers in said multi-carrier signal is carried out;wherein prior to said step of using: a power of both said centralcarrier and a scattered pilot neighboring said central carrier areidentified; and said power of said residual carrier perturbing saidcentral carrier is calculated by subtraction of said power of saidscattered pilot neighboring said central carrier and said power of saidcentral carrier.
 4. The method of claim 3 wherein said power of saidresidual carrier is expressed in terms of said power of said neighboringscattered pilot used in said subtraction.